1. Field of the Invention
The present invention is directed towards selecting and providing one of a plurality of clock signals.
2. Description of the Related Art
When data is retrieved from a medium, the data is often interpreted with reference to a clock. The clock provides an indication of the frequency at which the data is being provided and the instances at which the data is switched. Devices that are retrieving the data often utilize the clock when interpreting the data.
In some cases, a device may retrieve data from several mediums, and each medium may be providing data having an associated clock. The device may select to only receive data from one of these mediums at any given time.
FIG. 1 depicts a repeater set 100 in accordance with the Institute of Electrical and Electronics Engineers ("IEEE") 802.3u standard for communications networks. The IEEE 802.3u standard is set forth in IEEE Standards for Local and Metropolitan Area Networks: Supplement to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, Media Access Control (MAC) Parameters, Physical Layer, Medium Attachment Units, and Repeater for 100 Mb/s Operation, Type 100BASE-T (Clauses 21-30), The Institute of Electrical and Electronics Engineers, Inc., 345 East 47th Street, New York, N.Y. 10017-2394, 1995, which is hereby incorporated by reference.
The repeater set 100 includes a repeater unit 101 coupled to a plurality of physical layer devices 102. Each of the physical layer devices 102 is coupled to a medium 103, such as a Category 5 unshielded twisted pair cable, which is typically coupled to either data terminal equipment (not shown) or another repeater set (not shown). The interface between each physical layer device 102 and medium 103 may conform to a medium dependent interface ("MDI") specified by the IEEE 802.3u standard.
Upon receiving data from a medium 103, a physical layer device 102 recovers a receive clock from the incoming receive data stream and provides the receive clock and the receive data to the repeater unit 101. The repeater unit 101 performs the function of receiving data from one of the physical layer devices 102 and retransmitting the data to each of the other physical layer devices 102 coupled to the repeater unit 101. Each of the other physical layer devices then transmits this data to its respective medium 103.
The repeater unit 101 provides transmit data to each of the physical layer devices 102 through a set of signals forming a transmit channel (TX.sub.1, TX.sub.2, TX.sub.3, and TX.sub.N). Each physical layer device 102 provides the repeater unit 101 with a set of signals forming a carrier channel (CAR.sub.1, CAR.sub.2, CAR3, and CAR.sub.N) to indicate that valid data is being received by the physical layer device 102 from its respective medium 103.
Each physical layer device 102 provides the data received from its respective medium 103 to the repeater unit 101 via a receive channel (RX.sub.1, RX.sub.2, RX.sub.3, and RX.sub.N). Each receive channel includes a receive clock signal (RX.sub.-- CLK) and receive data signal (RXD). The repeater unit 101 uses the receive clock to sample the receive data from the physical layer device 102. The physical layer device 102 provides an indication signal on its carrier channel, indicating that it is providing data from a medium 103 on its receive channel. In response, the repeater unit 101 selects the receive clock from that physical layer device 102 to sample the data that is being provided on the receive channel. The repeater unit 101 then retransmits the data to all of the other coupled physical layer devices 102 for transmission to their respective mediums 103.
The IEEE 802.3u standard provides limitations on the amount of time that may elapse from an instance that incoming data arrives at the MDI of a receiving physical layer device 102 until an instance that the same data is transmitted onto the MDI's of all the other physical layer devices 102 in the repeater set 100. Accordingly, it is desirable for the repeater unit 101 to be able to rapidly select the receive clock from the receiving physical layer device 102, so that the repeater unit 101 can begin propagating the receive data through the repeater set 100.
However, the repeater unit 101 operates asynchronously with respect to the receive channel from each physical layer device 102. As a result, if the repeater unit 101 merely selects a receive clock when one is desired, the receive clock may be provided at any instance within the period of the receive clock's cycle. If this moment is close to an edge transition, the repeater unit 101 may be provided with only a small pulse (spike). Such a spike may result in a metastability or other corruption in the sampling of data from the selected receive channel. Accordingly, it is desirable for a mechanism for selecting a receive channel, including a receive clock, to provide for reducing the probability that spikes will occur in the selected receive clock.